The field of the invention relates generally to wafer chucks, and more particularly to flat wafer chucks for uniform thermal contact and methods of producing the same.
Some semiconductor processes, such as photoresist removal (ashing), require a relatively high wafer temperature to achieve the desired process result. For such thermally driven processes, the wafer temperature uniformity determines the ash rate uniformity across the wafer. Ashers are high throughput machines because ashing is one of the most frequently used processes. Generally, a bulk ashing process takes from 10 to 15 seconds, depending on the photoresist, while the overhead steps, such as wafer transfer, pump-down, wafer heating, process pressure stabilization and venting, take just about as much time or more. For this reason, overhead time is just as crucial as process time to the throughput of the machine. Wafer heating is the only overhead step that not only affects the machine throughput but also affects the process result. Fast and uniform wafer heating improves overall machine throughput and the process.
Non-uniform wafer heating introduces another devastating problem: wafer warping. Heating up wafers too rapidly can cause wafers to warp if the heating is not uniform. Unbalanced thermal stresses in wafers due to rapid and non-uniform heating forces a wafer to adjust its shape to find an equilibrium. It takes a wafer more than 10 seconds to relax back to its original flat shape according to our experience. Wafer warping slows down the machine substantially and is considered to be unacceptable in the semiconductor industry.
One of the most direct and efficient ways to heat up a wafer is direct contact heating via a heated wafer chuck. For a 250xc2x0 C. chuck, the high initial heat transfer rate can cause the wafer temperature to rise from room temperature as fast as 100xc2x0 C./sec. The heat transfer slows down as the wafer temperature asymptotically approaches the chuck temperature, as shown in FIG. 1. It usually takes about four to five seconds for the wafer and the chuck to reach effective equilibrium. Another advantage to using a chuck to heat up a wafer is that it can employ open-loop heating. There is no need to use a wafer temperature sensor in order to know when the wafer has reached the desired temperature. Overheating a wafer (temperature overshoot) is effectively avoided by using a constant temperature chuck.
To obtain a uniform heat transfer and to prevent wafers from warping, the chuck should be very flat at high temperatures and the temperature distribution across the chuck surface should be uniform. If the chuck is not flat enough, non-uniform wafer heating occurs because a better heat transfer occurs at the high spots where wafer contacts the chuck and poorer heat transfer occurs at the low spots. A uniform temperature distribution can be easily achieved by using a high thermal conductivity metal such as aluminum alloys for the chuck material. A flat chuck can be easily machined with high precision CNC tools at room temperature, but that does not ensure its flatness at high temperatures.
Resistive heater elements are commonly used in a heated chuck. A resistively heated chuck can be readily manufactured and machined at room temperature to be extremely flat. When heated up, however, the expansion of the heater elements and the internal stress by machining and assembly can distort the chuck. Machining a metal part at high temperatures, particularly at the chuck""s intended working temperature, is difficult because most metals become soft at high temperatures. Aluminum, for example, is practically impossible to machine at 250xc2x0 C. Most chucks are made of aluminum alloy because of its good thermal conductivity. Using aluminum as the chuck material makes the heater design relatively simple. No special heater pattern is required in order to obtain a uniform temperature distribution on the chuck surface because of aluminum""s high thermal conductivity. On the other hand, aluminum alloy also has a high coefficient of thermal expansion, which tends to alter its original shape when heated up. Distortion at high temperature causes the flatness to change and results in non-uniform wafer heating.
There are expensive solutions to achieve uniform wafer heating and to prevent wafer warping. Electrostatic chucks (xe2x80x9ce-chucksxe2x80x9d) and vacuum chucks (xe2x80x9cv-chucksxe2x80x9d) introduce additional forces to clamp down wafers and therefore are able to provide fast and uniform heating without wafer warping. Although these chucks are not new to the semiconductor industry, they are expensive. E-chucks are complicated and are the most expensive chucks to manufacture. And since they require other ancillary parts and systems, the reliability of e-chucks is always in question. Vacuum chucks are also expensive to manufacture due to the need for sealed vacuum channels inside the chuck.
Metal heater chucks are traditionally made in two ways: mechanical assembly and cast-in. In the case of mechanical assembly, the heater or heaters comprise two clamped and secured metal parts. FIG. 2 shows a cross-section of such a chuck 10, with an upper part 12, a lower part 14 and intervening heater element(s) 16. The least expensive method is to use screws to bolt both metal parts 12, 14 together. Arc welding or brazing is sometimes used alternatively to join the upper and bottom chuck parts 12, 14, but such methods are expensive.
In the case of a cast-in chuck 20, as shown in FIG. 3, a die is fabricated and molten aluminum or other metal is then poured into the die in which a heater element or heater elements 26 had already been placed in position. Secondary machining of the wafer-supporting surface after the molten metal is solidified produces the requisite surface finish. Conventional aluminum alloy used for casting chucks is porous and therefore not suitable for use in a vacuum chamber. Special sealing techniques or casting materials are required to avoid the problems associated with porosity. Cast-in chucks are extremely expensive if only a small quantity of chucks is to be produced, due to the tooling cost of the die.
Cable and tube heaters are two commonly used resistive heaters for chucks. They are basically resistive heating elements embedded inside a sheath made of stainless steel, Inconel(trademark) or other corrosion-resistant alloys to withstand the aggressive semiconductor processing environment. They can be easily bent to shapes that cover most of the chuck area to provide a uniform temperature distribution. They are widely available and can provide very high heating power at a very low cost. FIG. 4 shows a spiral shaped heater 20a and a serpentine shaped heater 20b as examples, though many other shapes are also possible. While illustrated as cross sections cast-in chucks 20 similar to that of FIG. 3, the skilled artisan will appreciate that such shapes can similarly be fitted into the grooves of the two-piece chuck 10 of FIG. 2.
When using cable or tube heaters, conventional wisdom suggests that a good physical contact between the heater and the chuck is required to heat up the chuck efficiently. Traditionally, the heater grooves of mechanically assembled chucks are designed slightly undersized and the heaters are press-fit in the grooves to obtain good physical contact. Press-fit introduces mechanical stress in the chuck such that the chuck surface is no longer flat after assembly. Secondary machining is often applied after assembly to restore the surface flatness. However, the surface flatness at room temperature does not mean that the chuck stays flat at high temperatures. Firstly, the thermal expansion of different materials (heater, fasteners, and chuck) introduces additional stress in the chuck, changing the chuck flatness. Secondly, the built-in mechanical stress during assembly starts to distort the chuck as the temperature rises.
In order to reduce such stress, a chuck 30 such as shown in FIG. 5 provides a top part 32 and a bottom part 34 on either side of a heater element 36. The top and bottom parts 32 do not contact each other except at screws 38 (one shown). The screws 38 are used with bevel or spring washers 39 to provide a spring force to secure the heaters 36. This design reduces the built-in mechanical stress, as the screws 38 are not fully tightened. One major drawback for this design is that, as the chuck 30 heats up, the temperature gradient from the heater(s) 36 to the chuck surface introduces thermal stress. The temperature is always higher in the area closer to the heater and lower at both the top and bottom surfaces. The thermal stress in the top part 32 and the bottom part 34 do not cancel each other since the top and bottom parts 32, 34 are not firmly secured to one another.
Accordingly, there is a need for improved wafer chucks for semiconductor processing.
In accordance with one aspect of the invention, a substrate chuck is provided for semiconductor processing. The substrate comprises a first part having a groove therein and a second part configured to tightly fit with the first part and thereby enclose the groove of the first part. A heater element is enclosed within the groove. Preferably, a clearance gap is left between the heater element and the surfaces defining the groove.
In accordance with another aspect of the invention, a method is provided for producing a chuck for supporting a substrate during thermal processing. The method includes assembling two parts with a heater element therebetween. The assembled chuck is thermally treated. Following thermal treatment, a supporting surface of the chuck is machined.